Pulse or digital communications – Receivers – Particular pulse demodulator or detector
Reexamination Certificate
1998-08-31
2001-10-16
Pham, Chi (Department: 2631)
Pulse or digital communications
Receivers
Particular pulse demodulator or detector
C375S262000
Reexamination Certificate
active
06304618
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to digital communications, and more particularly to reception of co-channel signals in digital communications.
BACKGROUND OF THE INVENTION
Bandwidth is a valuable resource in wired and wireless communication systems. Frequency may be reused in a wireless network in order to reduce cost. A signal occupying the same bandwidth as a desired signal, referred to herein as a co-channel signal, may cause interference and may severely limit the performance of a conventional single-user receiver. Moreover, a transmitted signal may travel along several paths to arrive at the receiver in a wireless environment. This multipath propagation may also give rise to signal fading and inter-symbol interference.
The effects of Co-Channel Interference (CCI) and Inter-Symbol Interference (ISI) can be reduced by using joint equalization/interference cancellation at a receiver. Among the various approaches, which include linear filtering and decision feedback, Joint Maximum likelihood Sequence Estimation (JMLSE) of co-channel signals may provide superior performance. See for example, Ranta et al, “
Co
-
Channel Interference Cancelling, Receiver for TDMA Mobile Systems
”, IEEE ICC Proceedings, February 1995, pp. 17-21. However, Ranta's approach may only be applicable to stationary channels (stationary over a data burst). Unfortunately, radio signals in mobile communication systems generally undergo Doppler shift caused by vehicular motion. When data bursts are long, this generally results in time-varying signal fading. Yoshino et al. propose joint adaptive channel estimation and demodulation in Yoshino et al., “
Interference Canceling Equalizer
(
ICE
)
for Mobile Radio Communication
”. IEEE Trans. Vehicular Tech., Vol. 46, No. 4, November 1997, pp. 849-861.
The receivers proposed in the above-mentioned publications obtain co-channel impulse response estimates jointly with the aid of training sequences of synchronized co-channel signals. In current Time Division Multiple Access (TDMA) cellular radiotelephone systems like GSM and D-AMPS, co-channel signals are not synchronized in either link. Thus, joint training may not be readily used for channel estimation. However, it is possible to exploit the training sequences of individual signals. One such technique which utilizes the training sequences of all signals is described in Wales, “
Technique for Cochannel Interference Suppression in TDMA Mobile Radio Systems
”, IEEE Proc. Comm., Vol. 142, No. 2, April 1995. This technique is also generally applicable to stationary channels.
Giridhar et al. propose adaptive joint MLSE and MAPSD algorithms which can estimate channel responses blindly without requiring any training in Giridhar et al., “
Nonlinear Techniques for the Joint Estimation of Cochannel Signals
”, IEEE Trans. Comm., Vol. 45, No. 4, April 1997, pp. 473-483. They use a technique called Per-Survivor Processing (PSP) which involves parallel channel state feedback in the recursion of an MLSE processor. PSP can provide an effective alternative to conventional adaptive algorithms that employ a single channel estimator. However, the acquisition performance of PSP-based MLSE receivers may have limitations as described in Chugg, “
Acquisition Performance of Blind Sequence Detectors Using Per
-
Survivor Processing
”, IEEE VTC Proceedings, Vol. 2, May 1997, pp. 539-543.
In a TDMA system, each co-channel signal may be subject to an overall channel impulse response which comprises a transmit pulse-shaping filter, a dispersive medium and a receive filter matched to the transmit pulse-shaping filter. The transmit and receive filter responses are known a priori and can thus be used to constrain the overall channel impulse responses. In the case of asynchronous co-channel signals, ringing arising from sampling offsets may increase the number of channel taps to be modeled. Estimation error generally rises as the number of taps being estimated increases.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide improved receivers and receiving methods that can reduce and preferably cancel co-channel interference.
It is another object of the present invention to provide receivers and receiving methods that can reduce co-channel interference without requiring an undue increase in the number of channel taps to be modeled.
These and other objects are provided, according to the present invention by estimating multiple timings for a received signal and sampling the received signal in accordance with the multiple timings, to produce multiple sample streams. The multiple sample streams are then used to detect information symbols. By generating and using the multiple timings and samplings of the received signals, efficient detection of information symbols may be obtained, without requiring undue increase in the number of taps that are being estimated. Reduction and preferably cancellation of co-channel interference can thereby be obtained.
More specifically, according to the present invention, a plurality of timings are estimated for a received signal wherein the plurality of timings correspond to a plurality of transmitted signals. The received signal is then sampled in accordance with the plurality of timings, to produce a plurality of sample streams from the received signal. Channel estimates are produced for the plurality of transmitted signals, and metrics are computed using the sample streams and the channel estimates. Information symbols corresponding to the transmitted signals are detected by using the metrics. Accordingly, by using multiple timings for a received signal, rather than using a common timing, the number of channel taps that are used may be reduced and the accuracy of symbol detection may be increased.
The multiple timings may be estimated for the received signal by correlating the received signal to at least one symbol sequence. Alternatively, a timing may be hypothesized and metrics may be computed that are associated with the hypothesized timing. The sampling may obtain a single sample per symbol period of the received signal in accordance with the multiple timings. Alternatively, the sampling may obtain multiple samples per symbol period of the received signal in accordance with the multiple timings.
The channel estimates may be produced by generating pulse-shape information and producing channel estimates for the multiple transmitted signals using the received signal and the pulse-shape information. Alternatively, channel estimates may be produced that correspond to a plurality of symbol sequence hypotheses. Channel estimates may be updated using an error signal.
The metrics may be computed by filtering the sample streams with the channel estimates to provide multiple filter outputs. One filter or multiple filters may be used. The filter outputs may then be used to compute the metrics. Alternatively, the metrics may be computed by estimating received values using the channel estimates and the hypothesized symbols, and computing the metrics using the estimated received values.
Finally, the information symbols may be detected by storing path information that is associated with a plurality of paths, and extending the paths by hypothesizing symbol values and computing metrics. Paths may be discarded based on the metrics or based on the metrics and a state space.
According to another aspect of the invention, a receiver can reduce the number of channel taps to be modeled by using pulse-shape information for the received signal. When the pulse shape information is used, a common timing and sampling may be used. However, as described above, the pulse-shape information may be used with multiple timings and samplings as well. Pulse shaping information may be obtained from the amount of ringing that is included in a received pulse. By using the pulse shape information, channel taps may be reduced.
More specifically, pulse-shape information is generated for a received signal. Channel estimates are produced for a plurality of transmitted signals using the received signal an
Bottomley Gregory E.
Hafeez Abdulrauf
Molnar Karl J.
Ericsson Inc.
Myers Bigel & Sibley & Sajovec
Pham Chi
Phu Phuong
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